This invention relates to semiconductor lasers, and in particular to laser modules with wavelength stabilization.
Modern optical network systems often require laser light of extremely precise wavelengths. For example, lasers emitting light at 980 nm and 1480 nm are used as pump sources for erbium doped fiber amplifiers, and in some cases are desirably accurate within a range of +/xe2x88x920.25 nm regardless of variations in power and temperature of the laser. In order to achieve such accuracy, the laser emission wavelength is stabilized by use of an external mirror in the form of a periodic perturbation in the index of refraction of the optical fiber pigtail coupled to the laser, also known in the art as a fiber Bragg Grating. The grating reflects a portion of the light back to the laser cavity and causes the laser emission to lock onto the desired wavelength and become relatively insensitive to power and temperature fluctuations. (See, e.g., U.S. Pat. No. 5,563,732 issued to Erdogan and incorporated by reference herein.) It is also desirable to control the output power of the laser. This is usually done by monitoring the light from the backface of the laser, converting the light into an electrical signal and utilizing this signal in a feedback loop to control the laser bias.
Under certain operating conditions of temperature and power, the feedback from the grating can cause instabilities, or ripple, in the fiber output power and in the backface monitor current resulting in non-linear characteristics as illustrated in FIG. 1. To make matters worse, the ripple in the fiber power (curve 10) and the ripple in the monitor current (curve 11) are out-of-phase, making it difficult to achieve constant power with conventional backface monitoring and feedback circuitry.
Some suggestions for avoiding this problem include use of front facet light for monitoring the power, and redesign of the feedback circuitry. Both approaches could involve costly and time consuming redesign of the modules.
A further problem which exists is the fact that locking of the laser emission to the grating is limited to a certain range of operating temperature and power. It is desirable to extend this range.
The invention is an apparatus including a semiconductor laser device emitting light at least at a certain wavelength, and at least two reflectors external to the device positioned to reflect the light to stabilize the wavelength.